Production of aluminum and its alloys

ABSTRACT

A PROCESS IS DISCLOSED WHEREBY ALUMINUM AND ITS ALLOY IS PRODUCED FROM OXIDIC MATERIALS, E.G. COAL SLAG OR OTHER LOW-GRADE ALUMINUN-CONTAINING MATERIALS, BY MIXING CARBON WITH THE STARTING MATERIAL IN AN AMOUNT IN EXCESS OF THAT REQUIRED FOR REDUCING THE OXIDES. A MELT IS THEN FORMED OF SAID MATERIAL WHICH IS APPLIED AND MAINTAINED OVER THE WALL SURFACE OF A REACTION ZONE IN THE FORM OF A CONTINOUS PROTECTIVE COATING WHILE CONTACTING THE MELT WITH ALUMINUM TRIHALOGENIDE GAS AT A TEMPERATURE ABOVE 1300*C. THIS COATING INHIBITS THE CORROSION OF SAID WALL SURFACE. THE ALUMINUM SUBHALOGENIDE GASES WHICH FORM IN THE REACTION ZONE ARE REMOVED, CONDENSED AT A TEMPERATURE BETWEEN 600 AND 1000*C. AND METALLIC ALUMINUM AND ALLOYS THEREOF ARE RECOVERED FROM THE CONDENSATE.

M1527, 1914 L. KAPOLYI e 3,832,164

PRODUCTION OF ALUIIHUI AND ITS ALLOYS F1105 larch 29, 1971 7 9 7 9 L j i 6 6 WI 11 l 8 8 Fig.1

United States PatentOffice 3,832,164 Patented Aug. 27, 1974 Int. (:1. ozzb 21/02 US. Cl. 75-68 B Claims ABSTRACT OF THE DISCLOSURE A process is disclosed whereby aluminum and its alloys is produced from oxidic materials, e.g. coal sl-ag or other low-grade aluminum-contain ing materials, by mixing carbon with the starting material in an amount in excess of that required for reducing the oxides. A melt is then formed of said material which is applied and maintained over the wall surface of a reaction zone in the form of a continuous protective coating While contacting the melt with aluminum trihalogenide gases at a temperature above I300 C. This coating inhibits the corrosion of said wall surface; The aluminum subhalogenide gases which form in the reaction zone are removed, condensed at a temperature between-600 and 1000 C., and metallic aluminum and alloys thereof are recovered from the condensate.

- This invention relates to a process and apparatus for the production of metallic aluminum and aluminum alloys, more particularly it is concerned with the recovery of aluminum from low grade ore and from coal slag not used before for this purpose.

On account of the large requirements of space, time and energy of the well-known Bayer-process for the manufact-ureof aluminum oxide and of the electrolysis of aluminum oxide other processes for the production of aluminum have been developed recently which involve lower investment. One of the drawbacks of the Bayerprocessis that it is suited for the processing of high grade aluminum ores. One of the aims of the development of ,newtechnologies was that materials of low grade i.e.

low A1 0 content could also be used as starting materials. One of these is the Pechiney-process where aluminum oxide is heated by coal and converted into aluminum and aluminum monoxide. After being conducted over a coalbed it is converted into aluminum carbide and finally by thermal decomposition metallic aluminum is recovered. In the similarly well-known Alcoa-process the aluminum oxide is reduced by aluminum carbide. In the Alcanprocess after the reduction of aluminum oxide by coal to aluminum and aluminum carbide metallic aluminum is produced with the help of the so-called subhalogenidereaction. The heart of the subhalogenide-method is that under suitable temperature and pressure conditions a reaction is brought about between the aluminum halogenides and the elementary aluminum or aluminum carbide leading to the formation of aluminum-subhalogenide. After its removal from the system aluminum-trihalogenide is formed again under cooling to ambient temperature and pressure conditions and metallic aluminum separates out. After all it could be said that the distillation of aluminum is carried out with the help of aluminum-trihalogenide as an intermediate.

A number of processes for the industrial implementation of the aluminum-subhalogenide method are known. For example German Pats. Nos. 261,162, 812,118 and 1,067,602 and US. Pats. Nos. 2,470,305, 2,762,702, 2,937,082, 3,243,282 and 3,351,461. According to these patents a reaction is brought about between contaminated aluminum or aluminum alloys and aluminum-trihalogenide gases leading to the formation of aluminum-subhalogenides from whichafter condensationmetallic aluminum is recovered.

A common feature of the processes outlined above and it is also their drawback that very high temperatures, above 2000 C., have to be used, generally in an electric arc furnace, for the conversion into aluminum carbide and consequently high heat-losses are incurred. A number of additional technical difiicu'lties occur. Under the joint effect of the temperature and the use of aluminum-trihalogenide extremely corrosive conditions arise, because in consequence of the thermal decomposition nascent chlorine is developed during the process.

In accordance with French Pat. No. 995,082, British Pat. No. 642,240 and US. Pat. No. 3,284,189 it is proposed to use for the reduction of the corrosive effect materials prepared from graphite, SiC, BN, TiC, A1 0 Si N or from combinations thereof everywhere where aluminum-'halogen'ides make contact with the equipment. On account of the drawbacks of the processes outlined above they could not be applied on an industrial scale, they are not saving on the one hand on account of the need for expensive materials and on the other on account of the expenses caused by the high temperatures.

Materials such as coal slag or other low-grade aluminum ores that are regarded as Wastes cannot be used as starting materials in the manufacture of aluminum and aluminum containing materials e.g. coal slag and low grade aluminum ores that are regarded as wastes, cannot be solved economically according to the state of art.

In the process according to the present invention molten aluminum-oxide containing starting material such as coal slag if necessary after previously removing iron therefrom, is introduced into a reactor made from customarily used materials. In the process the fuel content of the slag can also be utilized. The melt which forms a protective layer on the wall of the reactor is brought into reaction in the presence of carbon dust with aluminumtrihalogenide vapours. A greater part of the aluminumcontent of the melt is converted into aluminum-subhalogenide from which metallic aluminum is recovered in a known way.

Inthe process according to the present invention starting materials with low aluminum content, e.g. of 10% .Al O -content may be processed economically in the form tent of the starting material is removed prior tomelt formation by a carbo-thermal iron-removal process or by any other suitable reduction of the iron content (Fe O of the melt below a value of 2%. This is a necessary step when the iron content of the starting material is in excess of half of its A1 0 content.

' I The procedure for the removal of iron from low-grade aluminumores 'having a higher iron COI'lifiIltflOl from'coal slag is carried out in a primary reduction zone by cokereducti-on of the iron compounds with coke preferably at a temperature of 12001700 C. and removing the pig iron from the system. Then, the melt having a low iron content is conveyed to a zone where carbon dust is admixed with it .in excess with the help of a reduction burner and of compressed air or with oxygen enriched compressed air. On burning part of the injected carbon dust to carbon monoxide the solidification of the melt is inhibited and it is assured that the melt remains at the temperature necessary for the aluminum-subhalogenide reaction carried out later. The non-oxidized part of the carbon remains dispersed in the melt in an amount that if according to the invention A1Cl is used, the carbon necessary for the reaction should be present in a preferably twoor three fold-excess taking also into account the amount of carbon consumed by the Fe O and SiO content of the melt.

Experience indicates that carbon dust has to be injected into the system in an order of magnitude corresponding to that of the melt partly to assure the necessary temperature and partly to permit the reduction of aluminum oxide.

The combustion products are separated from the melt and after establishing an intimate contact between the melt which contains a suitable amount of carbon and aluminum-trihalogenide gases, a reaction takes place in the zone for the formation of aluminum subhalogenide gases, at a temperature above 1300 C., preferably at 1500- 1600 C. while on the inner wall of the reduction zone the melt forms a coating of 10-100 mm. thickness which is similar to a moving wall and is inhibiting corrosion of the materials of the reactor equipment. The aluminumsubhalogenide gases vented from the zone for the formation of aluminum-subhalogenides are cooled down in 1-5 stages and condensed in series connected condensing vessels at a temperature of 6001000 C. During this process at the different stages metallic aluminum of varying quality separates out from the aluminum-subhalogenide gases in the condensation vessels and is removed therefrom. The reconverted aluminum-trihalogenides are recycled into the zone for the formation of aluminumsubhalogenides. Advantageously the aluminum-subhalogenide gas is cooled and the gaseous impurities e.g. CO and CO are removed from the system. The solidified aluminum-trihalogenide is heated and fed back into the zone for the formation of aluminum-subhalogenides, simultaneously the lost aluminum trihalogenides are replenished. AlCl AlBr AlI and AlF aluminum trihalogenides can be used. In the zone for the formation of aluminum-subhalogenides-depending n. the composition of the starting material-not only aluminum-but also other subhalogenides are formed from the melt, especially the volatile subhalogenides of Si and Fe. These impurities present in the condensing vessel contaminate the condensing aluminum to varying degrees, with iron and silicon.

FIG. 1 shows the process according to the present invention for the case when the melt of the starting material is processed without previous removal of iron contamination of the raw material. The fused starting material is produced in a slag-smelting furnace. Molten coal slag 1) is conducted to zone (2) where injected coal dust (3) is admixed to it with the help of a reduction burner using compressed air or oxygen-enriched compressed air, then the melt mixed with coal (4)-is conducted into the zone (5) for the'formation of aluminum-subhalogenide and the melt is caused to flow down along the walls of the reactor towards the melt-outlet (8). In zone (5) the molten slag is preferably brought into contact in countercurrent with the aluminum-trihalogenide gases introduced through inlet (6).

A temperature of above 1300" C.

in the zone (5) brings -ab'out reaction of the mixture 'of 'molterrslag and carbon with the aluminum-trihalogenide vapour. The vapours of aluminum-subhalogenide which is formed in the reaction, are vented through outlet (7) into the condensing system (9). In the condensing system the vapours of the aluminum-subhalogenide decompose to metallic aluminum and to aluminum-trihalogenide. The aforesaid condensing system (9) comprises several-preferably three-condensing vessels connected in series. In these condensing vessels a temperature of 600-1000 C. is maintained and the vapours in the vessels are kept in intimate contact with the molten aluminum by bubbling them through the'molten metal. In, the zone'(-5) for-the formation of aluminum-subhalogenide a vacuumof 20- Torr is maintained. The molten metaland the molten metal alloys are respectively tapped through the outlets (11) of the condensing system (9). The aluminum-trihalogenide separating from the condensing system (9) is fed back through conduit (10) to the inlet (6).

If the aluminum-trihalogenide is cleaned by cooling, the gaseous contaminations, suchaslCO and CO may be separated out. In another step the 'aluminum-trihalogenide is reheatedpreferably in another vesselconverted-into vapour phase and recycled to inlet (6). The aluminumtrihalogenides are replenished through the charging inlet (12).

FIG. 2 shows another method for carrying out the invention in conjunction with the removal ofiron from the starting material. The .melt of the starting material, e.g. molten coal slag produced by the slag-melting furnace is conducted through a duct (1) to the iron removal zone (13) which is filled with hot coke. Heating of the coke is maintained by feeding oxygen and/or compressed air through a duct (14). 70-80% of the Fe O present in the melt is reduced on the heated coke to iron and the metal accumulated on the bottom of the reduction zone (13) is drained through the tap hole (16).

The slag melt having a low iron content is conducted from the reduction zone (13) through a conduit (15) to zone (2), where carbon-dust is admixed thereto. The further steps of the process may be carried out according to the process shown in FIG. 1.

One of the important features of the present invention is that in the zone for the formation of aluminum-subhalogenide the melt forms a protective layer on the wall of the equipment. FIGS. 3, 4 and 5 show examples for the solution of this task. According to FIG. 3 the formation of a protective melt layer is realized by' installing deflector tray-plates on the inner surface of zone (5) for the formation of aluminum-subhalogenides. The deflectors are disposed at an angle with respect to the flow-direction of the melt, preferably in an almost perpendicular array. On these deflecting tray-plates the melt is conducted dripping similarly to the well-known dripsmelting process. On the other hand the zone (5)for the formation of subhalogenide could be developed also in a way that on the inner surface the melt runs in a countercurrent with respect to the aluminum-trihalogenide gases.

According to the embodiment shown in FIG. '4 the melt is injected through spray equipment (18) into -the zone for the formation of aluminum-subhalogenides, consequently a considerable part of the melt impinges 'upon the wall of the equipment forming there a continuous film. The spray structure may be implemented e.g. by a spray nozzle.

The melt may be distributed on the inner wall'of the equipment by the injection of the aluminum-tr'ihalogenide gases so that part of the melt impinges upon the wall and part of it is sprayed in the inner space of the'appar'atus.

According to FIG. 5 the melt may form'a protective layer by keeping the body of the reactor for the formation of aluminum-snbhalogenides in a rotating motion about the axis of the reactor body. Consequently the melt 5 would be distributed on the-wall by the centrifugal force. The viscosityf the melt in zone for the formation of aluminum-subhaloge nide can be controlled by heating or cooling and is suitably maintained in a range of 30- 100 poise. In this manner the thickness of the protective melt-layer may beadjustegl. i

The advantageous f'eaturesof the present invention may be summed up as follows: on the one hand it becomes now possible to recover aluminum from coal slag which could not be utilized for this purpose in the prior art and from lower grade aluminum ores, on the other hand the heat energy of the coal slag as it leaves the furnace in molten state can also be utilized. A significant advantage is that the process of formation of aluminum-carbide and the utilization of a temperature of 2000 C. necessary to this purpose may be also eliminated. If desired, the iron content can be removed, also the loss on aluminum-trihalogenide is reduced and the production of aluminum from the aluminum-compounds present in the slag by processing with aluminum-trihalogenide may be carried out more effectively in presence of carbon at relatively low temperatures. Under the extremely corrosive circumstances the equipment can be made from customary construction materials because a protective layer is formed from the molten slag on its inner surface.

EXAMPLE 1 The coal used for combustion is carbon shale. Its heat content: 3500 Kcal., its slag content: 33%. The composi- The slag having the above composition and flowing from the cyclone furnace is conducted into the reduction zone which is filled with coke and heated by feeding into it oxygen or compressed air. Here 81% of the Fe O content is reduced into iron, and is discharged and removed. The molten slag is conducted into the reaction zone where carbon dust is injected with the help of compressed air through a reduction burner. The molten slag of 1600" C. temperature and containing free carbon is also injected into the zone for the formation of aluminum-subhalogenide. In this way part of the molten slag is brought to impinge on the wall of the reaction zone and flows towards the outlet, part of it remains suspended and is contacted by the A161 gases introduced through the bottom of the equipment. The thickness of the slag layer flowing down along the Wall can be regulated by varying the temperature inside the reactor.

The reaction zone for the formation of aluminum-subhalogenide is heated electrically, such as by a furnace. The molten, carbon containing slag reacts with the AlCl gas introduced in counter-current and the aluminum compounds are converted into aluminum-subchloride gases. The vapours of aluminum-trichloride and aluminum-subchloride which also contain C0, are conducted through a system of three condensing vessels Where-in order of sequencea temperature of 858 C., 745 C. and 647 C. respectively is maintained. In the reaction zone for the formation of aluminum-subhalogenide a vacuum of 50 Torr is maintained. The outer surfaces of the condensing vessels are made with heat-exchange ribbed sheets and the temperature may be regulated with the help of an air blower directed at them. As a result of the condensing process the first condensing vessel contains on its bottom a condensate of 93.4% A1 and 6.1% Si, the second condensing vessel a condensate of 87.3% Al and 11.2% Si and the third an alloy of 82.2% Al, 11.2% Si and 4.5%

Fe. The A1C1 gases'leaving the last condensing vessel are cooled down and the simultaneously present CO and CO gases are removed for further utilization. The aluminum-trichloride is reheated in a special vessel and by its own vapour-pressure refed into the reaction zone for the formation of aluminum-subhalogenide. The molten slag having a low ironand aluminum-content is removed from this reaction zone into a crystallizing furnace of e.g. 1284 C. temperature Where it is cast into moulds and may be utilized for the manufacture of artificial stones.

EXAMPLE 2 The liquid slag flowing from the cyclone furnace according to Example 1 and of identical composition is conducted into a reactor for the removal of iron, where the slag is kept molten by electrical heating. The iron oxide is reduced by coke and the metallic iron removed. The liquid slag of low iron content is mixed with coal-dust with the help of a reduction burner and compressed air and injected into the reaction zone for the formation of aluminum-subhalogenide at a temperature of 1550" C. The A1F vapours for the subhalogenide reaction are introduced in counter-current into the melt flowing down the walls of the reaction zone. In order that the melt should flow slowly along the walls of the reaction zone, deflecting trays prepared from silicon-carbide are mounted in an approximately perpendicular position to the direction of the flow of the liquid slag in order to bring about an intimate contact between vapours and melt. Electric heating is used to maintain the temperature at 1500-1550 C. The aluminum-subfluoride vapours are conducted into two series-connected condensing vessels situated on the upper part of the reaction zone in which temperatures of 940 C. and 810 C. are maintained in sequence.

In the first condensing vessel fused metal-alloy of a composition 87.5% A1 and 11.9% Si, in the second vessel an alloy having a composition of 82.7% A1 and 17.1% Si is recovered and condensed AlF is also present in the form of a powder. The C0 and CO gases are removed. The fused metals are discharged into the collecting vessel and the remaining AIF powder evaporated and recycled into the reaction zone for the formation of aluminumsubhalogeni-de.

What we claim is:

1. A process for the production of aluminum and its alloys, which comprises heating an oxide containing material of coal slag or other low-grade aluminum-containing materials, mixing carbon with the starting material in an amount in excess of that required for reducing the oxides in said starting material, forming a melt of said material, applying and maintaining the melt on substantially the entire wall surface of a reaction zone in the form of a continuous protective coating, while contacting the melt in said reaction zone with aluminum trihalogenide gases at a temperature above 1300" C. said coating thereby inhibiting the corrosion of said wall surface, removing the aluminum subhalogenide gases which form in said reaction zone therefrom, condensing said subhalogenide gases at a temperature between 600 C. and 1000" C., and recovering metallic aluminum and alloys thereof from the condensate.

2. The process of claim 1, wherein said step of contacting is carried out at a temperature between 1500 C. and 1600 C. under reduced pressure.

3. The process of claim 1, further comprising the first step of reducing any iron content in the starting material by reducing a substantial portion of its iron content to metallic iron, and separating the metallic iron from the starting material.

4. The process of claim 1, wherein said step of mixing carbon with the starting material comprises mixing coal dust through a reduction injection burner with the aid of compressed air or oxygen-enriched compressed air, and wherein the excess of carbon is a twoor threefold excess 7 8' over the amount which is stoichiometrically required for 3,615,359 10/ 1971 Toth '75--68 B reducing oxide content of the starting material. 3,615,360 10/1971 Toth et a1. '-75-68 R 5. The process of claim 1, wherein said aluminum trihalogenide is at least one of A101 AlBr A11, and AlF FOREIGN PATENTS 5 973,826 9/1950 France 75-68 B References Cited 582,579 11/1946 Great Britain 75 683, UNITED STATES PATENTS 734,480 8/1955 Great Brltaml l.. B 715,625 12/1902 Taddei 7568 B L. DEWAYNE R-UTLEDGE, Primary Examiner 2,723,911 11/1955 Phillips et a1. 7568B 2,783,991 3/1957 Morize B 10 L ANDREWS, Asslstant a n Y 3,292,914 12/ 1966 Southam 7568 B 

